The present invention relates to digital message transmission systems, and in particular to a digital message transmission system wherein two pulse sequence sequences are sampled at the transmit side at the rate of two plesiochronic data clocks, the sampled sequences being expanded by the insertion of additional information by "positive stuffing", and wherein the two pulse sequences including the additional information are combined to form a sum pulse which is transmitted to the receive side in a pulse frame, and wherein the procedure is reversed at the receive side for decoding the message.
In a digital message transmission system utilizing plesiochronic data clocks for sampling respective pulse sequences, the signals obtained by the sampling are again plesiochronic, that is, the sampled signals exhibit a slight frequency deviation relative to one another. In order to combine plesiochronic pulse sequences and to transmit the combined sequences to the receive side, the bit repetition rate of the sum pulse sequence must be at least as large as the maximum sum of the frequencies of the individual pulse sequences. When the frequencies of the individual pulse sequences are not at their upper limit, some bit locations in the sum pulse sequence are not required by the individual pulse sequences. The non-utilized bit locations must be eliminated at the receive side so that the individual pulse sequences at the receive side may in turn be reconstructed without error. To that end, additional information bits to be transmitted from the transmit side to the receive side must be inserted in the sum pulse sequence at the transmit side. Additionally, another information bit or bits is (are) inserted into the sum pulse sequence making possible synchronization of the receive side receiving the sum pulse sequence with the pulse frame of the sum pulse sequence. As a result of these additional bits, the bit rate of the sum pulse sequence increases slightly above a minimum value. Such methods of inserting additional bits in an existing pulse sequence are known as "pulse stuffing" methods or "positive stuffing" as described, for example, in the articles "Fundamentals of Communications Using Pulse Code Modulation," H. M. Christiansen and R. Kersten and "Pulse Code Modulation and Multiplex Equipment for 30 and 120 Voice Circuits," J. Gier, L. Schweizer, and R. Senft, both published in Siemens Review XLI (1974), Special Issue "Communications Engineering" respectively at pages 257-262 and 269-275.
It is necessary to co-transmit suitable phase information in the sum pulse sequence for specific systems designs working with plesiochronic data clock signals wherein the original transmit side clock bits of the plesiochronic data clock signals are required again at the receive side, and must therefore be regained from the incoming sum pulse sequence.